Method and apparatus for maintaining a water balance during impregnation and digestion of cellulosic material



June 28, 1966 Filed 0012. 14, 1963 G. H. TOMLINSON ll METHOD AND APPARATUS FOR MAINTAINING A WATER BALANCE DURING IMPREGNATION AND DIGESTION OF CELLULOSIC MATERIAL so 48 55 r50 FIG. I.

5 Sheets-Sheet 1 J1me 1966 e. H. TOMLINSON ll 3,258,390

METHOD AND APPARATUS FOR MAINTAINING A WATER BALANCE DURING IMPREGNATION AND DIGESTION OF CELLULOSIC MATERIAL Filed Oct. 14, 1963 5 Sheets-Sheet 2 June 1956 G. H. TOMLINSON n 8 3 0 METHOD AND APPARATUS FOR MAINTAINING A WATER BALANCE DURING IMPREGNATION AND DIGESTION OF CELLULOSIC MATERIAL Filed Oct. 14, 1965 5 Sheets-Sheet 5 FIG. 3.

United States Patent 60,58 5 Claims. (Cl. 162-17) This invention is related to theproduction of pulp by continuous digestion of wood or other cellulose raw material.

Traditionally, pulp has been commercially produced by submerging the chips in an excess of digestion liquor and using this excess liquor, which may be circulated throughout the chip mass, as a heat exchange medium. For instance, steam may be admitted to a zone near the bottom of the digester and the liquor so heated circulated by convection .to heat the chips. Alternatively, the liquor may be circulated by means of a pump through a heat exchanger external to the digester, and then return to the digester.

In conventional digestion the temperature and pressure of the digester are relieved to remove non-condensablegas. Also at the end of cooking, pressure is released over a short period of time. The steam given off during relief of the non-condensable and at the end of the cook, may be condensed and recovered as hot water.

It has been "known for some time from laboratory experiments that digestion can lie carried out by means of vapour phase heating. With this technique the chips are flooded with cooking liqu'or of predetermined concentration and after the chips are adequately impregnated, the surplus liquor is withdrawn. The initial concentration of the liquor is so adjusted that adequate chemicals will be retained by the chips to complete digestion following the removal of surplus liquor. The actual digestion is then carried out by heating the impregnated chips in a pressure vessel to a predetermined temperature where they are held until the digestion is completed, at which time the contents of the digester are cooled and the resultant pulp is separated from the residual liquor.

When attempts have been made to produce chemical pulps by the above procedure on a commercial scale it has been found that it is impossible to maintain a proper water balance. This results from the fact that the volume of the water contained in the digestion liquor, together with the chip moisture and condensate from the steam, normally is greater than the volume of fluid which the chips will absorb.

By means of the present invention the problem of water balance is now solved. Moreover means have been found to etiectively combine the digestion step with the washing and liquor evaporation stages in an unique manner thus improving the overall efiiciency of the process.

Thus the main object of the invention is to provide an improved system of maintaining a proper water balance in the material digested.

Broadly stated, the system of impregnation which is one of the main lfeatures of this invention includes a simultaneous evaporation of water to maintain such ratio of liquor to wood, that there will be no excess liquor over that required to completely impregnate the chips, this ratio being accurately and readily maintained in spite of variations in moisture content of the entering chips, liquor, etc.

Patented June 28, 1966 A further object is to provide an improved system "of impregnating, cooking, Washing and evaporating.

Another feature of this invention is the economy in chemical requirements as less chemical is required in the system to properly impregnate the chips.

Another object is to provide a system with greater overall heat economy. I

A substantial percentage of residual heat from this combined impregnating, cooking, washing and evaporating system is in the form of steam which can be used for other purposes. This may be contrasted with lower temperature vapour and liquor available from the batch operation. In addition, a reduction in the total heat exchange area is possible. Furthermore a high percentage of secondary heat from the system can be recovered in the form of hot water.

A further object of the invention is an improved washing system which allows a higher black liquor concentration at elevated temperature from the washing system.

Another feature of the instant system is the condensate recovery from which sufiicient water is available to supply all the demands of the washer, together with some of the water required in the black liquor recovery system.

Other objects and advantages will be evident from the following detailed description taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic illustration of one embodiment of the system.

FIG. 2 shows a system using a digester having an impregnation zone and a digestion zone and wherein liquor is flashed externally of the digester.

FIG. 3 schematically illustrates an alternative type of washer system.

Chips -fe'd form the chip storage bin '10 are first subjected to presteaming in the presteamer lal and then pass through a rotary valve 12 into an impregnation tube 13 partially [filled with liquor. Within the tube 13 the chips are impregnated at controlled pressure temperature, liquor concentration and time. The impregnated chips, free of residual liquor, then discharge through another rotary valve 14 to the cooking tube 15 where the chips are brought to temperature by means of direct steam through steam line 16 and the cooking cycle is carried-out at controlled temperature and time. As the cooking proceeds during passage through the tube 15 the chips shrink and black liquor is discharged from them; this discharged black liquor is removed from the coking tube 15 through the line 32.

From the cooking tube the chips pass through a third rotaryvalve 18 to tube :19 where they are carried by a screw conveyor 20, in the tube 119 a major portion of the heat is removed from the digested chips and the chips are partially washed by counter current contact with wash water from a further stage such as press washer 2&1. Washer 21 provides a second stage of washing of the washer system. The digested and washed pulp is recovered at high consistency through line 22.

IMPREGNATI'O'N AND WATER BALANCE By means of the present system of digestion all of the chemical required for the digestion of the chips is present in situ in the impregnated chips at the time they are brought to digestion temperature. Thus the digestion rate is controlled only by the chemical reaction rates and therefore digestion times can be short relative to conventional systems where a considerable portion of the chemical is outside the chip and the digestion rate is controlled by diflfus'ion. This shortened cooking time with the present system reduces undesired side reactions that occur in the liquor, and this together with the absence of recycled black liquor reduces the amount of chemical required. The reduced liquor to wood ratio during cooking with the present system also results in a residual liquor of materially higher concentration, thus reducing the amount of evaporation required.

Attempts to produce a chemical pulp on a commercial basis by vapour phase cooking have been frustrated by the fact that normally more water is supplied to the system (through the sap in the chips, the water in the digestion liquor and the condensate from pre-steaming the chips) than the chips will carry from the impregnation. I have found that a satisfactory water balance may be maintained by carrying out a variable amount of evaporation in the impregnation tube, the amount of evaporation depending on the moisture content of the chips and the make-up liquor concentration. This control is accomplished with the instant invention by superheating the re-circulating liquor a few degrees higher than the temperature maintained in the impregnation tube and allowing the liquor to flash either in the head of the tube or in a separate flash tank at some point after heating and before returning to the digester. By product process steam at approximately 15 to 75 p.s.i.g. corresponding to a temperature in the tube of 120160 C. is thus available for further use.

Many forms of i-mpregnator may be utilized, one type being illustrated schematically in FIG. 1. This impreg nator consists of a cylindrical impregnator tube 13 set at an angle to the horizontal and provided with a central midfeather dividing the tube in two sections. The angle of this tube is usually approximately 45 from the horizontal, but this angle may be varied within certain limits (approximately 30 to 60 to the horizontal). Chips are carried within the impregnator tube 13 by a controlled slow speed flight type conveyor or the like schematically indicated at 24 and are moved downward on the 'upper side of the midfeather 23 and upward along the lower side of said midfeat'her.

Liquor is circulated through the impregnator tube '13, via line 25 to a heat exchanger 26 and back to the top of the tube 113, through line (29. The heat exchanger is heated by high pressure steam from steam line 27. To maintain the concentration of the circulated liquor, make-up liquor is added from line 28, and the now fortified liquor is heated in heat exchanger 26 and recirculated through line 29 to the impregnation tube 13 which is held at a predetermined pressure by a pressure control .valve 30 in line 30. This fortified liquor heated to a predetermined temperature higher than the equilibrium temperature corresponding to the impregnation tube pressure enters, through line 29, the top of tube 13' above the level of the liquid therein and a portion thereof is flashed to steam. Steam thus produced is discharge-d at a predetermined pressure through the pipe 30.

impregnated chips are carried by conveyor 24 to a point above the liquid level within the impregnation tube 13 and discharged through a rotary valve 14.

A relatively constant level of liquid is maintained in the impregnation tube, which is held at the desired preset pressure, by control of the temperature in the recirculation liquor. Should the liquid level in tube 13 fall below a predetermined point less steam is supplied to the heat exchanger 26 with a resultant lowering of the temperature of the recirculation liquor which in turn reduces the amount of steam formed during flashing. This means that a greater portion of water remains in the liquor and accordingly the level of liquor in the tube 13 tends to rise. Conversely should the level rise above a predetermined point more heat is supplied to the heat exchanger 26 to increase the temperature of the recirculation liquor and thus cause greater evaporation when the liquor is flashed t-o therefore lower the level L. The above described level control may be operated automatically by suitable means sensing level L and controlling the heat supplied to the heat exchanger 26 while the pressure is held constant.

The impregnation liquor may be controlled to maintain proper chemical concentration either manually or, if suitable equipment is available, automatically, by adjusting the amount of make-up liquor added in line 28 to give the desired concentration. The concentration of the active cooking ingredient in the impregnation liquor is thus controlled to a predetermined value so that the chips leaving the impregnator will carry the correct amount of chemical to allow the digestion in the following stage.

Furthermore, by adding the concentrated make-up liquor to the relatively dilute liquor being re-circulated to and 'from the impregnator, the chips are never subject to the damaging action of the high concentration liquors.

It will be noted that the conveyor speed is variable and therefore the time of feed through the impregnation tube may be varied as desired. The temperature maintained in said tube may be any suitable impregnation temperature.

-It is also noted that the pressure of process steam removed via pipe 30 corresponds to the vapour pressure of the pool of liquor maintained in the impregnation tube. The temperature of said pool can also be controlled to the proper impregnation temperature for the chips.

A second impregnator system incorporated into a different type of diges-tor than that illustrated in FIG. 1 is shown in FIG. 2. In the system of FIG. 2 impregnation and digestion are carried out in one vessel while flashing occurs externally of the vessel. As illustrated, the flashing system has been incorporated on an up flow digester wit-h impregnation occurring in the lower zone in the digester.

Liquor is withdrawn from the impregnation zone below the level L in the digester \100 through line 25A and then proceeds through a heatexchanger 26A where it is heated by steam from line 27A. Fresh chemical is added to the system via line 2 8A. The amount of heat applied in heat exchanger 26A is controlled in accordance with the level L"in the digester 100 in the same manner as the heat exchanger 26 is controlled in accordance with level L as described hereinabove.

Heated and fortified liquor is carried via line 29A into a flash tank 13A. The flash tank is held at a pre-set pressure as was the impregnator 13, so that water will be flashed in the tank 13A. The amount of water flashed depends on the difference in temperature between the heated fortified liquor in line 29A and the liquor in the flash tank. Steam generated by flashing of liquor in tank 13A is removed via line 30B which would be connected .to the evaporator 44 as is line 30 of the first embodiment. A pressure regulator valve 30C controls the pressure in the flash tank 13A.

After flashing within the tank 13A liquor is returned to the impregnation zone via line 2913. The liquor level L in the flash tank is held constant by controlling the flow through line 29B by means such as valve 29C. Thus, if more liquid is flashed in tank 13A more steam is removed in line 3013 and less liquor remains in the tank which tends to lower the level L Lowering of the level L reduces flow through line 29B which thus lowers the level L in the digester as less liquid enters the digester.

In the above manner, the amount of flashing in tank 13A adjusts the chemical concentration of the fortified liquor and the control of flow through line 29B controls the amount of liquor circulated to the digester.

The present system of temperature control to flash a variable amount of water from circulated liquor may generally be applied where it is desired to hold the liquor to wood ratio at some value lower than that which would otherwise prevail as a result of water entering the system with the chips, the steam and the make-up liquor. Thus this method may be applied regardless of the design .of the digester, or the direction of chips flow (i.e. up,

down, or at an angle) and the steam generated by the flashing may be usefully applied for secondary purpose in a mill system. I

Thus applicant has disclosed a system of controlling the water balance by providing for a variable amount of evaporation in the system. This fact allows a constant liquor to chip rat-i as the chips pass through a constant environment of controlled chemical concentration, at controlled temperature and during a controlled time.

COOKING AND WASHING Referring back to FIG. 1, after the chips have been impregnated they descend through rotary valve 14 into the top of the cooking tube 15 as herein-before described. The cooking tube 15 is of similar construction to the impregnation tube 13 and is provided with a flight type conveyor schematically indicated at 31. As in the impregnator tube 13 the chips are carried downward along the top of tube 15. High pressure steam is injected into the cooking tube through a steam line 16 to maintain a predetermined cooking temperature. Cooking time is controlled by the rate of travel of the conveyor 31, which may be varied to obtain proper cooking of the chips being digested.

With this cooking system wherein cooking is carried out in a separate section in a vapour phase with the chips being continuously fed therethrough along an incline, black liquor which drains from the chips during cooking may be easily drained from the cooking tube to black liquor line 42. A high percentage (approximately 50%) of the total water content entering the cooking tube is removed in this manner, and at a solids content of approximately 30%. Thus with this system the capacity of the washers may be reduced since a large quantity of black liquor is removed in the cooking tube 15. The draining from the cooking tube is fully described in co-pending Canadian application No. 880,584 by I. Bryce.

If desired a puddle of liquor may also be maintained in the tube 15 by controlled drainage. It is also possible to circulate some of the black liquor from the tube 15 reduced and the solids content of the black liquor in tube 15 would be increased. This liquor drained through line 32 is at still higher concentration than when direct steaming per se is used.

Digested chips exit at the top of the cooking tube 15 and pass through a rotary valve 18 into a diffusion washer schematically illustrated at 19. In the washer 19, which operates at a pressure approaching that of the digester, the chips are carried .by the conveyor 20 in counter current flow to washing liquor entering through the pipe 37. The chips after passing through washer 19 may then be fed to the inlet of a press washer 21 where a second washing step is performed. The thus digested and washed stock is removed from the system via line 22.

The difiusion washer 19 consists essentially of a tube mounted at approximately the same angle to the horizontal as the impregnation tube or the cooking tube and has a screw type conveyor 20 for carrying the material upwardly through the washer as shown. Washing liquid is admitted toward the top of the washer by pipe 37 to proceed in counter flow to the material being carried by the conveyor 20.

' Black liquor is removed through the strainer 19 adjacent the top of the vertical section 19A of the washer 19 and is conducted away via line 41.

By moving the digested stock and wash water in counter-current flow with the wash water entering the washer proceeding downward and then upward, black liquor at relatively high concentration and temperature is available in line 41. The pulp from the cooking stage is both cooled and washed in the washer 19.

In the illustrated embodiment wash Water from pipe 38 enters the press washer 21 with the digested chips through line 39. The liquor from the press washer 21 is conducted via line 37 to the upper part of the diffusion washer 19 to proceed in counter flow to the digested stock. It may be emphasized that all the wash liquid required is available as evaporator condensate obtained from a subsequent stage of the system. A dilferent vform of washing system is illustrated in FIG. 3. In this system a pool of liquor 69 is maintained in the tube 68 in the line leaving the cooking tube. The stock from the cooking tube passes through the pool 69 and through valve 18 or the like into the line 61. The line 61 conducts the stock to tank 62 from whence it proceeds via line 63 to a conventional rotary drum washer 64 the vat of which is at atmospheric pressure. A portion of the liquor carries with the digested chips through valve 18 and is removed at the washer 64. Liquor returned to tube 68, and in excess of that passing through valve 18, rises in tube 68 in counter-current flow to the digested chips thus increasing its concentration to some extent while at the same time increasing its temperature to about 300 F. to provide a hot pool of liquor. This heated liquor drains from pool 69 through strainer 67 and line 66. The liquor in line 66, while still maintained at digester pressure is combined with the liquor drained through pipe 62 directly from digestion tube 15, and this combined flow of liquor is conducted via line 33 to heat exchanger 42 where the incoming digestion reagent is pre-heated. The pre-heated incoming digestion re-agent then passes via line 28 to the impregnation system.

By the methods illustrated in FIGS. 1 and 3 a major portion of the heat in the digested chips is extracted by contacting them, while at digester pressure, with liquor at a lowertemperature obtained from a subsequent washing stage'. This allows discharge of the chips from the pressure zone at a temperature which will prevent evolution and rapid expansion of vapour within the chip thus reducing the damage usually incurred by blowing pulps.

At the same time the temperature of the liquor obtained HEAT RECOVERY SYSTEM A continuous source of process steam from the impregnator tube 13 is readily available for any useful purpose. "In the illustrated embodiment the process steam ttrom the impregnator tube 13 is conducted via the line 30 to a first evaporator 44, for concentrating black liquor.

The supply of steam to evaporator 44 is supplemented, if necessary, by steam from line 30A which may be obtained from the boiler. Vapour from evaporator 44 is conducted to a second evaporator45 through a line 46 while condensate from said first evaporator 44 feeds to a flash tank 48 via the line 47. To increase the vapour supply to the second evaporator 45 the steam from flash tank 48 is conducted through the line 49 into said evaporator 45.

The vapour from the second evaporator 45 provides a source of low pressure steam (approximately 5 p.s.i.g. for

a system in which the pressure of secondary steam from the impregnator tube 35 p.s.i.g.) in the line 50. Con- ,densate from said evaporator is recovered as hot water .and is conducted away via line 51. The hot water thus recovered is supplemented with the condensate from flash 'tank 48 via line 52 to provide sufiicient hot water to supply all the demands for the Washing system as well as to provide surplus for use in causticizing.

With the washing system illustrated in FIG. 1 black liquor at a high temperature (in excess of 212 F. and probably between about 290 F. and 320 F.) provides a convenient source of heat which may easily be recovered in line 41. Black liquor at digestion temperature and pressure drained from the tube 15 in line 32 is added to the liquor in line 41 to supplement the heat source. In the embodiment of FIG. 1 black liquor from lines 32 41 are conducted via line 33 while still maintained at elevated pressure to a heat exchanger 42 which serves to heat the make-up liquor supplied to the system. From heat exchanger 42 the black liquor proceeds to a flash tank 43. Vapour from said flash tank 43 is available for further use, for example, in the presteamer 11.

Black liquor from flash tank 43 is fed via line 53, oxidization tower 54, if required, and line 55 into evaporator 45 and from evaporator 45 to evaporator 44 through line 56. In some processes an oxidation tower may be dispensed with and the residual liquor fed directly from the heat exchanger to the evaporator 45. The evaporated liquor from evaporator 44 enters flash tank 58 through line 57 and the steam produced in tank 58 is added to the low pressure steam in line 50 through pipe 59. Concentrated black liquor from tank 58 is conducted by line 60 to a suitable recovery system for incineration of chemicals and recovery of heat.

Because applicants impregnation system allows a lower liquor to chip ratio the black liquor concentration from the washers is materially higher than with conventional systems. The type of washers used will also contribute to the high concentration. This high black liquor concentration reduces the amount of evaporation required and therefore the use of two efiects of evaporation (in addition to the evaporation in the impregnation tube) is possible. In conventional systems five effects are usually required with the accompanying higher heat requirements. Applicants two stage system provides a source of low pressure steam (5 p.s.i.g. for an incoming steam pressure of 35 p.s.i.g.) while a five stage system provides no steam for recovery (the steam from the end of a five stage system would be at a negative pressure, approximately 28" vacuum).

A further source or heat economy is available from the incineration of chemicals as there is less inorganic material present and therefore less heat is carried away.

It is evident that with applicants system a material reduction of heat requirements is possible.

The following is a specific example of a heat balance obtainable with the disclosed system illustrated in FIG. 1 operating on a krvaft process. All weights, unless otherwise specified, -are given in lbs/ton pulp unbleached 90% dry basis.

Chips are steamed in the presteamer 11 using 700 lbs. of vapour for 6820 lbs. of moist wood containing 2730 lbs. H O as moisture, and this wood along with 3430 lbs. or" water at 212 F. pass on to the impregnation tube 13. A temperature of 282 F. is maintained in the impregnation tube 13 by re-circulating liquor heated to a temperature of 311 F. by the heat exchanger 26. This re-circulating liquor has a ratio 15 to 1 in this example (15 parts recirculation liquor to 1 part make-up liquor) with the liquor entering the impregnator at a concentration of 33.7 gums/litre of eflective Na O and removed therefrom at 30.5 guns/litre with the added make up liquor having a concentration of 90 grms./litre of eflect-iv'e N a O at a temperature of 282 F.

It will be noted that even though the make-up liquor has a concentration of 90 grms./ litre, the chips are never subject to liquors of a concentration above 33.7 grrns./ litre in this example. This feature of the present invention ensures that the chips are not damaged by contact with high concentration liquor.

Process steam at 35 p.s.i.g. at 2400 lbs./ ton generated in the impregnator as a result of superheating liquor as the dittusion washer proceeds to a press washer.

previously described is available from the impregnation tube in line 30 for reclaiming.

The temperature of the incoming re circulating liquor is adjusted to give the amount of evaporation required, to maintain a proper level of liquid in the impregnation tube.

I-mpregnated chips are carried through valve 18 to the cooking tube which in this example is kept at a temperature of 365 F. using 1135 lbs. of high pressure steam. 4920 lbs. of black liquor at 365 F. drain from tube 15 via line 32.

1840 lbs. of pulp and 4920 lbs. of liquor proceed through the diflusion washer in counter-current flow to the wash water. This pulp and the black liquor left after The resultant pulp removed from the system by line 22 contains 1840 lbs. of pulp, approximately 70 lbs. of

- black liquor solids and 1770 lbs. of water and is at temperature of 220 F. The black liquor leaving washer 19 through line 41 is at a temperature of 297 F.

Process steam in line 30 is supplemented with 1200 lbs. of steam at 35 p.s.i.g. from the line 30A from a boiler or other suitable source, to supply a total of 3600 lbs. of steam to the first evaporator 44.

Exiting from the evaporator 44 are 3320 lbs. of steam evaporated from the black liquor at 251 F. and 3600 lbs. of condensate. The steam from evaporator 44 is fed to evaporator 45 while condensate from said evaporator 44 is flashed in tank 48 to produce lbs. of steam at 251 F. and 3490 lbs. of condensate. The steam from flash tank 48 is also fed to evaporator 45.

Evaporator 45 produces .2510 lbs. of process steam at 227 F. by evaporation of black liquor and 3430 lbs. of condensate. This condensate is added to that from the flash tank 48 for a total of 6920 lbs. of condensate at 251 F. which is passed in heat exchange with incoming cold water andthen 5840 lbs. of said condensate at 200 F. is supplied to the washer via line 38 with the remainder of the condensate going to the recovery system for use in dissolving the smelt.

As above stated 5840 lbs. of condensate is supplied to the washer system from which 8920 lbs. of liquor at 297 F. are removed in line 41. The 4920 lbs. of black liquor drained from the digested chips are available at 365 F. in line 32 and this liquor is combined with the liquor in line 41 to give a total of 13,840 lbs. of black liquor at 322 F. which is fed to the heat exchanger 42 via line'33. The temperature of the black liquor leaving the heat exchanger is reduced to 286 F. and the black liquor is then flashed in tank 43 to produce 860 lbs. of steam, some of which is fed to the presteamer. 12,980 lbs. of black liquor from flash tank 43 proceed through an oxidation tower 54 from which 11,690 lbs. of liquor at 164 F. is fed to the series evaporators 45 and 44 and finally to flash tank 58. In flash tank 58, vapour in the amount of 220 lbs. is produced which is added to the process steam from the second evaporator 45 for a total of 2730 lbs. of process steam at 5 p.s.i.g. 5720 lbs. of black liquor feed from flash tank 58 via line 60 to a standard recovery system.

The process steam used in the heat exchanger 26 to heat the recirculating liquor for the impregnator resulted in 2600 lbs. of condensate being produced.

Table 1 given below is a comparison of the heat economy of the disclosed continuous digestion system just described with a batch system. Unbleached spruce Wood Cl'llPS were used in this comparison.

TABLE 1 .the level of the chipinlet.

TABLE 1Continued B 5. Black liquor solids concentration to multiple effect evaporator 24.6 16.0 6. No. of conventional evaporator effects 2 5 7. Steam generated from liquor (less recycled at furnace) 14,800 14,490 8. Steam to digester 3,740 5,100 9. Steam to evaporator 1,200 3,200 10. Total steam required for cooking and evaporating 4,940 8,300 11. Net high pressure steam fromrecovery furnace after cooking and evaporating 9,900 6,190 12. Low pressure secondary steam from I evaporator 2,730 13. Total steam from system ..1..... 12,630 6,190

A-Continuous digestion by method of present invention.

B0onventional batch digestion.

From the above table it is evident that the steam requirements with the instant invention are materially reduced over that of a conventional system. This reduction in heat requirement is mainly due to the lower amount of chemical carried by the chips. As previously pointed out more heat is available from the furnace since there is less inorganic material in the smelt to carry the heat away. Less heat is also required for evaporation since applicants system can operate with only two stages of evaporation and some heat may be recovered from the e'vaponation system.

Example 1 herein below demonstrates one mode of operation of the continuous digestive system disclosed using the kraft process and includes a description of the pulp produced in this example, the washing systems disclosed were not used.

Example 1 Spruce chips, containing 40.5% moisture were fed at a rate of 2.42 lbs. per minute of moisture-free wood to the presteaming tube of a pilot plant continuous digester similar to that shown in FIGURE 1. Steam was admitted to the tube at a rate sufficient to maintain a pressure of p.s.i.g. The condensate formed was drained from the bottom of the tube. The conveyor speed was set so that the residence time of the chips in the presteaming vessel was 5 minutes.

The presteamed chips were introduced into an impregnating tube in which a liquor level was maintained up to The chips were carried through this vessel by means of a flight conveyor, the speed of which was maintained to give a residence time of 30 minutes for the impregnation. The concentration of effective alkali in the feed liquor was maintained at 32.6 grams per liter by adding fresh liquor to liquor which was being recycled through the circulating line. In order to maintain this concentration, a liquor flow of 0.5 Imperial gallon per minute at a concentration of 84.0 grams per liter was established. The fresh liquor was made up to a sulfidity of 30% while the liquor in the impregnating tube reached equilibrium at a sulfidity of 45%. The liquor in this tube was maintained at a temperature of 136 C.; and a pressure of 30 p.s.i.g. The liquor was superheated to a temperature of 145 C. in a heat exchanger in the circulation and allowed to flash to 136 C. so that a water balance was maintained in the impregnation tube.

Following impregnation, the chips were introduced into the cooking vessel'which was held at 150 p.s.i.g. by the addition of direct steam. The retention time of the chips in the cooking zone was 30 minutes. Condensate and the liquor which drained from the chips as the digestion proceeded were removed from the bottom of the digester, the cook being carried out essentially in the vapor phase. At the end of the cooking period the chips were 10 removed from the digester through a rotary valveand-allowed to flash to atmospheric pressure.

A sample taken from the pulp showed the following characteristics Chlorine No. 4.5 C.E.D. viscosity 15.6 GB. brightness 29.7 Screen reject (percent of pulp) 1.76

The pulp was beaten in the Valley Beater and showed the following strength characteristics:

Beating Time (min.) 0 46 67 Canadian Standard Freeness (ec.) '642 450 300 Bulk (cc/gm.) 1. 59 1. 34 1. 27 Tensile Breaking Length 8. 5 12.5 13. 0 Tappi Burst Factor 60 88 81 Tappi Tear Factor 157 108 102 Burst plus Tear 139 142 132 MIT Fold .1 900 1, 970 2, 540

In a cook carried out in a similar manner but using a liquor of sulfidity, pulp samples were obtained from the digestor by blowing in the regular manner and also by cooling the digested chips to 212 F. before discharging. The following table illustrates a comparison of the cold discharge with the hot blow.

From the above comparison it is evident thta the characteristics of the cold discharged pulps are better than those from a hot blown pulp. Thus with applicants cold discharge system fibre damage and strength deterioration are reduced when compared with the hot blow discharge.

A higher percent of heat from the system is in the form of high pressure steam than in conventional batch system. Also less total heat exchange area is required for digestion and evaporation than is the case for conventional batch operation and a high percent of secondary heat from the disclosed system can be recovered.

The impregnator and method of impregnating have been disclosed in relationship with pulp digestion systems. This method and apparatus could also be used in other environments for impregnation of materials which carry variable amounts of water and in which the impregnant is carried in water or other vaporizable liquid.

In some extreme circumstances no evaporation need take place in the impregnation tube. For example, should not enough Water enter the impregnation tube the level of the pool of liquor therein would have to be controlled by injecting water into the impregnation tube. This probably would only happen if dry chips, such as from kiln 'dry wood were fed to the impregnator. Under these circumstances controlled amounts of water are added to the impregnator in accordance with the level L sensed to maintain the predetermined level L in the tube. The liquor recirculated through heat exchanger 26 would be heated to maintain the temperature in the pool of liquor, but because of the shortage of water little or no steam would be removed from the impregnator. The amount of water added may be controlled by sensing the level andcontrolling the water added to maintain level L substantially constant. Liquor concentration and temperature, the pressure in the impregnation tube and the time raw material and as water with the digesting chemical and leaves said system with the impregnated raw material, a method of impregnating said cellulosic raw material while maintaining a water balance in the system comprising: maintaining a pool of impregnation liquor in an impregnation zone, continuously feeding said cellulosic raw material to said pool, introducing heated impregnation liquor to said impregnation zone, said heated impregnation liquor being of a temperature higher than the temperature of liquor in said pool, controlling the temperature of said heated impregnation liquor to cause a portion thereof to flash and form process steam, the amount of said heated impregnation liquor flashed being such as to maintain the level in said pool substantially constant and then removing said process steam from the system.

2. In the continuous pulping of cellulosic raw material wherein said raw material is subjected to impregnation with an impregnation liquor containing an aqueous solution of digestion chemical in an impregnation system thereby to impregnate said material with digesting chemical in a predetermined weight ratio and wherein water enters said impregnation system as moisture with the raw material an as water with the digestion chemical and leaves said system with the impregnated raw material, a method of impregnating said raw material while maintaining water balance in said system comprising: maintaining a pool of impregnation liquor at substantially constant temperature in an impregnator, continuously feeding raw material to said pool, continuously removing liquor from said pool, indirectly heating said removed liquor to a temperature above said temperature in the impregnator, introducing said removed liquor into a zone of substantially constant pressure thereby to cause a portion of said removed liquor to flash, maintaining a relatively constant level in said pool by controlling the amount of heat applied to said removed liquor in accordance with the level in said pool, thereby to control the amount of said removed liquor flashed and'then removing process steam formed by said flashing from said system.

3. An apparatus for use in continuous pulping of cellulosic raw material wherein said raw material is subjected to impregnation with an impregnation liquor containing an aqueous solution of digesting chemical in an impregnation system, thereby to impregnate said material with digesting chemical in a predetermined weight ratio and wherein water enters said impregnation system as moisture with said raw material and as water with the digesting chemical and leaves said system with the impregnated raw material, said apparatus comprising; an impregator having a pool of impregnation liquor therein and a zone of controlled pressure above the level of said pool, circulating means for circulating impregnation liquor from said pool through an indirect heating means and back to said impregnator, means controlling the temperature of said circulating liquor in accordance with the level in said pool to cause a controlled amount of said impregnation liquor to flash in said zone of controlled pressure, thereby to maintain said level substantially constant, and means for conducting process steam produced by said flashing away from said impregnator.

4. An apparatus for use in continuous pulping of cellulosic raw material wherein said raw material is subjected to impregnation with an impregnation liquor con taining an aqueous solution of digesting chemical in an impregnation system thereby to impregnate said material with digesting chemical in a pre-determined weight ratio and wherein water enters said impregnation system as moisture with said raw material and as water with the digesting chemical and leaves said system with the impregnated raw material, said apparatus comprising; a vessel having a pool of impregnation liquor therein, a flash tank, indirect heating means, means for circulating impregnation liquor from said pool to said indirect heating means and then to said flash tank, means for conducting impregnation liquor from said flash tank back to said pool, means for controlling the temperature of the impregnation liquor entering said flash tank in accordance with the level in said pool to cause a controlled amount of water to flash in said flash tank thereby to maintain the level in said pool substantially constant and means for conducting process steam produced by said flashing from said flash tank.

5. An apparatus for use in continuous pulping ofcellulosic raw material wherein said raw material is subjected to impregnation with an impregnation liquor containing an aqueous solution of digesting chemical in an impregnation system thereby to impregnate said material with digesting chemical in a pre-determined weight ratio and wherein water enters said impregnation system as moisture with said raw material and as water with the digesting chemical and leaves said system with the impregnated raw material, said apparatus comprising; a digester vessel, a flash tank, indirect heating means, said vessel having an impregnation zone filled with impregnation liquor and a digestion zone, means for conducting impregnation liquor from said digester vessel to said indirect heating means and then to said flash tank, means for conducting impregnation liquor from said flash tank to said digestor vessel, means for controlling the flow from said flash tank to said digester vessel While maintaining a substantially constant level of impregnation liquor in said flash tank, means for controlling the temperature of the impregnation liquor entering said flash tank in accordance with the amount of impregnation liquor in said digester vessel to cause a controlled amount of water to flash in said flash tank thereby to maintain the amount of impregnation liquor in said vessel substantially constant and means for conducting process steam from said flash tank.

References Cited by the Examiner UNITED STATES PATENTS 6/1940 Dunbar 162-239 10/1957 Durant et a1 16252 

1. IN THE CONTINUOUS PULPING OF CELLULOSIC RAW MATERIAL WHEREIN SAID RAW MATERIAL IS SUBJECTED TO IMPREGNATION WITH AND IMPREGNATION LIQUOR CONTAINING AN AQUEOUS SOLUTION OF DIGESTING CHEMICAL IN AN IMPREGNATION SYSTEM THEREBY TO IMPREGNATE SAID MATERIAL WITH DIGESTING CHEMICAL IN A PREDETERMINED WEIGHT RATIO AND WHEREIN WATER ENTERS SAID IMPREGNATION SYSTEM AS MOISTURE WITH THE RAW MATERIAL AND AS WATER WITH THE DIGESTING CHEMICAL AND LEAVES SAID SYSTEM WITH THE IMPREGNATED RAW MATERIAL, WHILE MAINTAINING A WATER BALANCE IN THE SYSTEM COMPRISING: MAINTAINING A POOL OF IMPREGNATION LIQUOR IN AN IMPREGNATION ZONE, CONTINUOUSLY FEEDING SAID CELLULOSIC RAW MATERIAL TO SAID POOL, INTRODUCING HEATED IMPREGNATION LIQUOR TO SAID IMPREGNATION ZONE, SAID HEATED IMPREGNATION LIQUOR BEING OF A TEMPERATURE HIGHER THAN THE TEMPERATURE OF LIQUOR IN SAID POOL, CONTROLLING THE TEMPERATURE OF SAID HEATED IMPREGNATION LIQUOR TO CAUSE A PORTION THEREOF TO FLASH AND FORM PROCESS STEAM, THE AMOUNT OF SAID HEATED IMPREGNATION LIQUOR FLASHED BEING SUCH AS TO MAINTAIN THE LEVEL IN SAID POOL SUBSTANTIALLY CONSTANT AND THEN REMOVING SAID PROCESS STEAM FROM THE SYSTEM. 